Mixer does not copy -spec's

[zkessin]

As of now mixer does not copy in -spec() lines into the target files, the problem is that this means that the compiler warning "warn_no_spec" can't be used with mixer.

[seth]

Copying over dialyzer specs is a nice idea. I may not have time in the near future to investigate a patch, but would welcome a PR.

[zkessin]

Any ideas on how to do it? I would be happy to play with it, if given a pointer in the right direction